1. Field of the Invention
The present invention relates to a method and tools by which serrations and threads which are different in rolling diameter from each other are formed simultaneously on a shaft member (workpiece) by rolling.
2. Description of the Prior Art
One of the examples of shaft members having serrations and threads different in rolling diameter from each other is a steering shaft of an automobile as shown in FIG. 1. In this figure, serrations 2 and threads 3 are formed successively in one end portion of a steering shaft denoted by 1. A steering wheel 4 having serrations in the inside diameter is fitted, with its phase determined in the rotational direction, to said steering shaft 1, and is fixed by a nut 5.
In order to form those serrations and threads different in rolling diameter from each other by rolling, such a method as shown in FIG. 2 has been applied generally, in which: serration rolling tool 6 and thread rolling tool 7 set in different positions and combined serially with each other are provided in a pair, and each part of the pair is moved in the direction of an arrow A toward a shaft member 9 supported by two centers 8.
The above-described relationship in arrangement between serration rolling tool 6 and thread rolling tool 7 requires a large rolling stroke and, consequently, a large rolling machine, which increases the cost of equipment. Moreover, according to the foregoing method, serrations and threads are formed, strictly speaking, in separate rolling processes, which causes other problems in that an improvement in productivity, the greatest advantage of rolling machining, and the reduction of cost cannot be fully effected.
For the purpose of overcoming the above problems, there has been a trial in which the aforesaid serration rolling tool 6 and the thread rolling tool 7 are arranged in parallel so that the rolling operation with those tools can be performed simultaneously. According to this method, however, extra axial tension is generated during the rolling operation, because a prescribed lead angle .alpha. (FIG. 2) of the thread rolling tool is formed in accordance with the elements in the form of a rolled article in this case, and said tension works so as to hinder rolling for forming serrations, in particular, thus causing a failure in forming a prescribed number of the teeth of a serration, including the lack or excess thereof. Therefore, this method as not been applied in practice.